This invention relates generally to gate valves and more particularly to gate valves having two valve discs for respectively closing the inlet and outlet ports of the valve, the valve discs being lowered along a vertical axis to a position between the ports and then being moved transversely of the axis to close the ports.
FIG. 1 is a schematic showing in perspective a double-disc gate valve which, with the exception of components 69 and 71, is of conventional design. Referring to FIG. 1, the conventional components of the valve include an elongated valve body, or casing, 1 whose lower portion encloses tubular seat bodies 3 and 5. The bodies 3 and 5, which respectively define valve ports 7 and 9, terminate respectively in opposed annular valve seats 11 and 13. As shown, process pipes 15 and 17 are respectively connected to the valve ports. Extending through the top portion of the casing 1 is a stem 19 whose external end is connected to suitable means for raising and lowering the stem. The lower end of the stem supports a wedge-shaped body 21. The wedge extends transversely of the stem and is positioned with its thinner edge downward. The ends of the wedge are provided with grooves 25 and 27 for slidably engaging longitudinally extending, opposed guide rails 23 on the casing, only one of these rails being in view in FIG. 1.
As shown, the slides of the wedge 21 are provided with upwardly extending grooves 29 and 31 for slidably retaining disc-spreaders 33 and 35 respectively, as by means of dove-tail joints. The spreaders are wedge-shaped and are disposed with their thinner edges upward; thus, longitudinal movement of the wedge relative to the spreaders moves the latter transversely in the casing. Valve discs 37 and 39 are rotatably coupled to the spreaders 33 and 35, respectively, and are positioned outwardly thereof. The coupling 40 between each disc and its spreader is designed with a small amount of play for facilitating mating of the disc and its seat. As shown, each spreader is provided with a central, downwardly extending tongue 59 having a lateral projection 61 (FIG. 2) which extends close to the face of the associated valve disc to stabilize the position of the disc. The tongue for the spreader 33 is not in view in FIG. 1. The outer face of the inlet valve disc 39 is provided with a sealing ring 41 for engaging the inlet valve seat 13; the outer face of valve disc 39 is provided with a similar sealing ring (not in view) for engaging the outlet seat 11.
Closing of the valve shown in FIG. 1 is effected by lowering the stem 19 from the position shown. The resulting downward movement of the gate assembly (i.e., the wedge-spreader-and-disc assemblies) is guided by the rails 23. Mounted to the inside wall of the casing are two pairs 43 and 45 opposed stops, only one member of each pair being in view. When the gate assembly is lowered to the position where the valve discs are in alignment with their respective seats, 11 and 13, the stops 43 intercept the ends of the spreader 33; similarly, the stops 45 intercept the ends of the spreader 35. Thus, continued downward movement of the stem 19 and wedge 21 forces the disc-and-spreader assemblies transversely outward, moving the spreaders 33 and 35 immediately beneath pairs 47 and 49 of opposed hold-down blocks, respectively, and moving the discs 37 and 39 into sealing engagement with their respective seats. As the outlet disc 37 nears its seat, a lug 65 carried by the top of the disc moves immediately under a hold-down block 67 mounted to the inside of the casing. The action of the various hold-down blocks is described in more detail below.
When the stem 19 is lifted to open the valve, the initial upward movement of the stem lifts the wedge 21 relative to the spreader-and-disc assemblies. Because the various hold-down blocks oppose upward movement of the spreader-and-disc assemblies, these assemblies are retracted transversely inward; thus, scraping between the sealing rings and valve seats is avoided. During retraction, the outlet disc lug 65 moves out from under its hold-down block 67, following which the spreaders 33 and 35 move out from under their respective hold-down blocks 47 and 49. Further upward movement of the stem lifts the gate assembly to the fully open position shown.
For about thirty years, large motor-driven gate valves have been in use in the United States gaseous diffusion plants for the separation of uranium isotopes. The gate valves originally installed in the plants were generally similar to the valve shown in FIG. 1, but were not provided with the components designated therein as 65, 67, 69, and 71. During the first several years of process use, it was found that the valves sometimes jammed when operated at relatively high pressure differentials. That is, sometimes jamming occured during opening of a gate valve whose inlet pressure exceeded its outlet pressure by more than a certain amount. This jamming typically resulted in severe damage to the valve, making necessary to remove a block of isotope-separation stages from the process stream for several hours while the damaged valve was removed and replaced. Typically, the jamming resulted in bending or breakage of the joint between the outlet spreader 33 and the wedge 21, as well as in occasional distortion or breakage of the wedge 21 near the guide rails 23. It was apparent that the inlet disc 39 was opening before the outlet disc 37 had opened, and that the resulting application of the full pressure differential to the outlet disc was preventing it from retracting without somehow cocking sufficiently to jam.
In an attempt to solve this problem, the diffusion-plant gate valves were modified in 1957 by providing the outlet discs with the above-described hold-down arrangement 65, 67. This arrangement was provided to supplement the hold-down action of the blocks 43 for the outlet spreader and thus ensure that the outlet disc retracted properly. This modification did not solve the problem, however, and since 1957 costly valve failures of the kind described have occurred from time to time, even in valves which had successfully passed static-load tests before installation. Because of such failures and because of interest in the use of higher operating pressures, studies of the jamming problem were made. As a result, various re-designs of the valve were proposed, but none of these was found acceptable. As an example, one proposed re-design of the outlet-disc assembly was based on the belief that during retraction the outlet disc turns somewhat about its vertical axis, assuming a cocked position causing jamming.